Growing research and development efforts are being made for semiconductor integrated circuits composed of field-effect transistors (hereinafter referred to as FETs) using compound semiconductor materials such as gallium arsenide (hereinafter referred to as GaAs) and aluminum-gallium arsenide (hereinafter referred to as AlGaAs). Typical of such compound semiconductor devices are GaAs Schottky gate transistors and selectively doped AlGaAs/GaAs devices. These heterojunction devices exhibit electron mobilities several times higher than those achievable by presently known silicon devices and have been expected to contribute to realizing faster semiconductor integrated circuits.
The fact is however that such GaAs and/or AlGaAs devices are, at least at the present stage of development, not satisfactory for achieving such high switching speeds that have been expected to be achievable by GaAs/AlGaAs devices. The inability of GaAs/AlGaAs devices to provide high switching speeds is pronounced especially at the LSI level and is attributable to the inadequate current driving capacity of the devices. Therefore, attempts are being made to realize semiconductor integrated circuits composed of bipolar transistors using GaAs or various combinations of GaAs and AlGaAs. Typical of such bipolar transistors is a heterojunction bipolar transistor (HBT), especially a wide band gap emitter HBT having an emitter region which is wider in band gap than the base region of the transistor. A prototype device of this kind of HBT will be the wide band gap emitter transistor disclosed by P. M. Asbeck et al in "(Ga,As)As/GaAs Bipolar Transistors for Digital Integrated Circuits", IEDM Tech. Digest 1981, 629-632. The HBT therein taught uses AlGaAs for the emitter and appears to be predominant over much of the current research and development efforts being made for HBTs.
However, most of presently known HBTs persistently have various problems to be solved. Among these problems is the complexity in the device configuration and in the process of device fabrication. The large collector-base capacitances and the limited operation speeds of such devices are also important ones of the problems to be solved.
An advanced semiconductor device is proposed in presently co-pending U.S. patent application Ser. No. 807,935 in which holes are injected into an active layer of undoped GaAs from a hole injector layer of p.sup.+ -type to induce a channel of electrons similar in effect to the two-dimensional electron gas layer formed in a known high electron mobility transistor (HEMT). The electrons induced in the active layer of the semiconductor device therein disclosed are trapped in the layer solely with the aid of the potential barrier intervening between the active layer and the hole injector layer and for this reason could not be reliably confined within the electron channel created in the active layer.